1. Field of the Invention
The present invention relates to a fixing device configured to fix an image onto a recording medium, and an image forming device including the fixing device.
2. Description of the Related Art
In various kinds of image forming devices such as photocopiers, printers, facsimiles, or MFPs of the foregoing devices, copies and records can be obtained by heating an unfixed image transferred to and carried on a recording medium such as paper so as to fix the image to the recording medium.
On fixation, the unfixed image is heated while the recording medium carrying the unfixed image is sandwiched and conveyed by a fixing member and a pressing member, thereby to fuse and soften a developer, in particular toner, contained in the unfixed image so as to penetrate the recording medium. Accordingly, the toner is fixed onto the recording medium.
When the fixing member is heated by a heat source to a predetermined temperature, if heating time until the predetermined temperature is reached is sufficiently short, it is possible to reduce significantly energy consumption without exerting great influence on ease of use even if a pre-heat process is eliminated in a stand-by state. To achieve the advantage, the fixing member is formed by low-heat capacity members or the like, such as a thin-walled roller and a thin-walled belt made of a metallic base material and an elastic rubber layer. In addition, the heat source is formed by a halogen heater heating the fixing member by radiation heat, a ceramic heater, an IH system with high heating efficiency, or the like, for realization of rapid heating. The fixing devices having these components are disclosed in Japanese Patent Application Laid-open No. 2007-79040, Japanese Patent Application Laid-open No. 2010-32625, Japanese Patent Application Laid-open No. 2007-334205 and Japanese Patent Application Laid-open No. 2008-129517, for example.
In the foregoing fixing devices, at execution of an image fixing operation, heat from the fixing member and the pressing member is absorbed by a recording medium, for example paper, passing through a nip portion between the two members; and thus the temperatures of the fixing member and the pressing member do not become abnormally high. However, when the fixing member and the pressing member do not rotate at a sudden stop of the image forming device due to occurrence of a paper jam or other events, during power-off of the image forming device, in a stand-by mode, in a low-power mode, or the like, a portion of the fixing member neighboring the heat source may be locally heated and excessively raised in temperature by residual heat of the heat source even if power supply to the heat source is stopped. In particular, if the fixing device includes the fixing member that is made lower in heat capacity by decreasing the wall thickness or the like for the purposes of shortening of a warm-up time and reduction of energy consumption, the fixing member tends to be excessively raised in temperature.
Meanwhile, these fixing devices are configured such that, if a temperature detection unit detects an excessive rise in temperature of the fixing member, an overheat protection circuit is activated to determine this state as abnormal; shut off power supply to the heat source; and stops rotation of the fixing member and the pressing member, thereby bringing the image forming device to an abnormal stop. In general, it is hard for a user to recover the image forming device from the abnormal stop made by the overheat protection circuit; and thus the user needs to ask technical personnel or the like from a device manufacturer to conduct a recovery operation. Accordingly, if the image forming device is brought into an abnormal stop by the overheat protection circuit, it takes a relatively large amount of time to complete a recovery operation and allow the image forming device to be operable again. The time is so-called “down time.”
For the reasons described above, the fixing member may be excessively raised in temperature within a predetermined period of time from a sudden stop or power-off of the image forming device due to occurrence of a paper jam or the like or from shift of the image forming device to the stand-by mode or the low-power mode or the like. Accordingly, if the image forming device is restarted within the foregoing predetermined period of time from a sudden stop or power-off of the image forming device, the temperature detection unit may detect an temporary excessive rise in temperature of the fixing member; and the overheat protection circuit may misjudge this state as abnormal and may stop again the image forming device. Similarly, the temperature detection unit may detect a temporary excessive rise in temperature of the fixing member within the predetermined period of time from shift of the image forming device to the stand-by mode or the low-power mode or the like, and the overheat protection circuit may misjudge this state as abnormal and stop the image forming device. In this case, there is a problem that the “down time” becomes relatively long until the image forming device becomes operable again.
There is a need to provide a fixing device that, when an image forming device is restarted after a sudden stop or power-off due to occurrence of a paper jam or the like, or when the image forming device shifts to a stand-by mode or a low-power mode or the like, can re-start operation or continue a predetermined mode safely and appropriately, without causing an abnormal stop due to misjudgment of the overheat protection circuit or the like, and an image forming device including the fixing device.